Nitric oxide is unique among the higher oxides of nitrogen in its reactivity and efficiency for the oxidation of oxygen-bound hemoproteins. Stoichiometric conversion of oxyhemoglobin or oxymyoglobin by nitric oxide to methemoglobin or metmyoglobin and nitrate occurs at rates that are similar to those obtained for the association of oxygen and nitric oxide with these hemoproteins. The mechanistic details for oxidations of hemoglobin and myoglobin by nitrite ion and alkyl nitrites have also been determined. These reactions are chracterized by alkyl nitrite or nitrous acid association with the hemoprotein even in oxygen saturated solutions. Stoichiometric conversion of two hemoglobin units to equivalent amounts of methemoglobin and nitrosylhemoglobin occurs upon treatment with either nitrite ion or alkyl nitrites in deoxygenated solutions. In oxygen saturated solutions, alkyl nitrites oxidize hemoglobin and myoglobin solely to their oxidized forms. Detailed analysis of nitrosyl transfer processes between coordinated cobalt nitrosyls and hemoglobin A has established that nitrosyl transfer is triggered by prior ligand association with the cobalt nitrosyl complex. Preliminary evidence suggests that the beta-93 cysteine residue is responsible for nitrosyl release.